The whole plant consists of a very long, much-branched, but undivided tubular filament. Where it is in contact with the substratum, root-like outgrowths are formed, not unlike those observed in _Vaucheria_. At first the walls are colorless, but later become dark smoky brown in color. A layer of colorless granular protoplasm lines the wall, becoming more abundant toward the growing tips of the branches. The spore cases, "sporangia," arise at the ends of upright branches (Fig. 32, _C_), which at first are cylindrical (_a_), but later enlarge at the end (_b_), and become cut off by a convex wall (_c_). This wall pushes up into the young sporangium, forming a structure called the "columella." When fully grown, the sporangium is globular, and appears quite opaque, owing to the numerous granules in the protoplasm filling the space between the columella and its outer wall. This protoplasm now divides into a great number of small oval cells (spores), which rapidly darken, owing to a thick, black wall formed about each one, and at the same time the columella and the stalk of the sporangium become dark-colored. When ripe, the wall of the sporangium dissolves, and the spores (Fig. 32, _E_) are set free. The columella remains unchanged, and some of the spores often remain sticking to it (Fig. 32, _D_). [Illustration: FIG. 32.--_A_, common black mould (_Mucor_), × 5. _B_, three nearly ripe spore cases, × 25. _C_, development of the spore cases, i-iv, × 150; v, × 50. _D_, spore case which has discharged its spores. _E_, spores, × 300. _F_, a form of _Mucor mucedo_, with small accessory spore cases, × 5. _G_, the spore cases, × 50. _H_, a single spore case, × 300. _I_, development of the zygospore of a black mould, × 45 (after De Bary).] Spores formed in a manner strongly recalling those of the pond scums are also known, but only occur after the plants have grown for a long time, and hence are rarely met with (Fig. 32, _I_). Another common mould (_M. mucedo_), often growing in company with the one described, differs from it mainly in the longer stalk of the sporangium, which is also smaller, and in not forming runners. This species sometimes bears clusters of very small sporangia attached to the middle of the ordinary sporangial filament (Fig. 32, _F_, _H_). These small sporangia have no columella. Other moulds are sometimes met with, parasitic upon the larger species of _Mucor_. Related to the black moulds are the insect moulds (_Entomopthoreæ_), which attack and destroy insects. The commonest of these attacks the house flies in autumn, when the flies, thus infested, may often be found sticking to window panes, and surrounded by a whitish halo of the spores that have been thrown off by the fungus. ORDER II.--WHITE RUSTS AND MILDEWS (_Peronosporeæ_) These are exclusively parasitic fungi, and grow within the tissues of various flowering plants, sometimes entirely destroying them.